How can I convert a buffer of a slice of bytes (&[u8]) to an integer?

I'd suggest using the byteorder crate (which also works in a no-std environment):

use byteorder::{BigEndian, ReadBytesExt}; // 1.2.7fn main() {    let mut buf: &[u8] = &[0, 0, 0, 1];    let num = buf.read_u32::<BigEndian>().unwrap();    assert_eq!(1, num);}

This handles oddly-sized slices and automatically advances the buffer so you can read multiple values.

As of Rust 1.32, you can also use the from_le_bytes / from_be_bytes / from_ne_bytes inherent methods on integers:

fn main() {    let buf = [0, 0, 0, 1];    let num = u32::from_be_bytes(buf);    assert_eq!(1, num);}

These methods only handle fixed-length arrays to avoid dealing with the error when not enough data is present. If you have a slice, you will need to convert it into an array.

See also:

• How to get a slice as an array in Rust?
• How to convert a slice into an array reference?

I'd like to give this answer here to commit the following additional details:

1. A working code snippet which converts slice to integer (two ways to do it).
2. A working solution in no_std environment.
3. To keep everything in one place for the people getting here from the search engine.

Without external crates, the following methods are suitable to convert from slices to integer even for no_std build starting from Rust 1.32:

Method 1 (try_into + from_be_bytes)

use core::convert::TryInto;let src = [1, 2, 3, 4, 5, 6, 7];// 0x03040506u32::from_be_bytes(src[2..6].try_into().unwrap());

use core::conver::TryInto is for no_std build. And the way to use the standard crate is the following: use std::convert::TryInto;.

(And about endians it has been already answered, but let me keep it here in one place: from_le_bytes, from_be_bytes, and from_ne_bytes - use them depending on how integer is represented in memory).

Method 2 (clone_from_slice + from_be_bytes)

let src = [1, 2, 3, 4, 5, 6, 7];let mut dst = [0u8; 4];dst.clone_from_slice(&src[2..6]);// 0x03040506u32::from_be_bytes(dst);

Result

In both cases integer will be equal to 0x03040506.

This custom serialize_deserialize_u8_i32 library will safely convert back and forth between u8 array and i32 array i.e. the serialise function will take all of your u8 values and pack them into i32 values and the deserialise function will take this library’s custom i32 values and convert them back to the original u8 values that you started with.

This was built for a specific purpose, however it may come in handy for other uses; depending on whether you want/need a fast/custom converter like this.

https://github.com/second-state/serialize_deserialize_u8_i32